Jimmy Savile is said to have committed his crimes ‘under the noses of BBC staff for more than 40 years’. Photograph: Murdo MacLeod

The BBC will be plunged into a major crisis with the publication of a damning review, expected next month, that will reveal its staff turned a blind eye to the rape and sexual assault of up to 1,000 girls and boys by Jimmy Savile in the corporation's changing rooms and studios.

Dame Janet Smith, a former court of appeal judge, who previously led the inquiry into the murders by Dr Harold Shipman, will say in her report that the true number of victims of Savile's sexual proclivities may never be known but that his behaviour had been recognised by BBC executives who took no action.

Smith's investigations, which followed the Pollard inquiry into why the BBC shelved a Newsnight programme about Savile, will send shockwaves through the corporation.

A source close to the inquiry told the Observer: "The numbers are shocking. Many hundreds and potentially up to 1,000 people were victims of Savile when he was representing the corporation. The report will overshadow Pollard. It will go right to the heart of how Savile was able to get away with the most heinous of crimes under the very noses of BBC staff for more than 40 years."

The sheer scale of victims' testimonies being examined has delayed the publication of Smith's report by a month.

Peter Saunders, chief executive of the National Association for People Abused in Childhood (Napac), which has been consulted by Smith's inquiry, said: "In Savile's lifetime I wouldn't doubt [that 1,000 people had been abused by him on BBC property]. The other thing I have found extraordinary, and very sad, is the number of people I have spoken to connected to the BBC, and that is a lot of people, who said: 'Oh yes, we all knew about him.'

"I was talking to someone at BBC Manchester in Salford who said 'we knew about Stuart Hall. He had a room where he would take women and young people'. You think: 'Oh my God, these people were offending almost in open sight and no one thought to intervene.'"

Liz Dux, a lawyer representing 74 of Savile's victims, said Smith had been forensic in her examination of witnesses and her report was likely to cause serious concerns for those at the top of the organisation. She said: "Every single opportunity Savile took it. He never had a quiet day basically so these numbers wouldn't at all surprise me.

"Dame Janet is very widely respected and I am confident she won't leave any stones unturned. The clients who gave evidence said that they felt they were listened to very sensitively and sympathetically and were able to give their evidence in a lot of detail. This will not be a what-the-BBC-want sort of report."

A second report on the scale of Savile's abuse within the NHS has also been delayed due to the number of places in which Savile committed crimes and it is not expected until June.

Smith has used a similar methodology to that employed during the Shipman inquiry, which found the GP had killed hundreds of patients, not just the 15 for which he received life sentences before taking his own life in his prison cell.

Her team sent letters to every member of BBC staff past and present asking whether they had witnessed criminal acts by Savile in order to piece together his pattern of behaviour and establish an understanding of the scale of his crimes.

In three known cases, one of which involved a BBC cameraman who has since died, Savile carried out his abuse with others connected to the corporation, the review has heard.

The report will, however, express frustration that some of those closest to Savile or culpable for allowing him to go unchallenged have refused to co-operate. His criminality peaked in the 1960s and 1970s, when he was middle-aged and at the height of his career at the corporation, but continued right up until the last filming of Top of the Pops in 2006 when at the age of 79 he groped a girl aged between 13 and 16. Smith's review has been in contact with more than 1,000 witnesses and victims, including the 138 who are pursuing civil claims for compensation, but the scale of those affected by Savile's crimes dwarfs the number who have so far come forward.

The Observer understands the BBC has provided more than £10,000 in funding, and the assistance of a business consultant, to Napac to allow it to increase its helpline services. Further money is expected to be made available when the review is published.

Lord Hall, the BBC's director general, met the charity's chief executive shortly before Christmas and asked for his support when the Smith report is launched.

Dux hopes the BBC will respond to Smith's findings by offering further support to the victims, who are due to receive limited compensation through a scheme being agreed with the corporation, the NHS and the Jimmy Savile Charitable Trust. Those raped by Savile are unlikely to receive more than £50,000 in compensation.

Dux, head of abuse cases at Slater & Gordon, said: "What I hope doesn't happen is that the BBC goes into some sort of navel-gazing period. Rather than look internally, look at how they are behaving and accept some corporate responsibility, which is not what they have done so far.

"I have asked for counselling for my clients who have given statements but the BBC have done nothing; my clients have been left absolutely high and dry."

If the BBC really cared about these people then they would have contacted them as soon as they have given evidence and said: 'We accept that you have gone through an awful ordeal and whatever the outcome of the report we have made facilities to let you go and see this counsellor.'"

She added: "Whether these cases are resolved by settlement scheme or by court the amount of damages the victims of the BBC will get is absolutely tiny compared to what they have spent on their own legal fees, the Pollard inquiry and their own staff. The damages for compensation in civil law for rape is rarely over £50,000 and that is something that is life-changing and hideous. They are actually getting an insulting amount".